Achieving high atomization and good surface quality in conventional air spray applications increases the prevalence of this application method. However, the underlying problems such as low transfer efficiency, high paint cost and high environmental pollution have caused to search for other methods. During these searches, atomization and the surface quality to be obtained are important. The following factors affect atomization quality:
- Paint viscosity (high viscosity=big particles) – depends on paint
- Surface tansion of paint (Low surface tansion=small particles) – depends on paint
- Air pressure (High air pressure=small particle) – depends on spray gun
- Nozzle diameter (Small nozzle diameter=small particle) – depends on spray gun
- Paint pressure (High pressure=small particle) – depends on spray gun
- Volume of air (high volume of air=small particle) – depends on spray gun
Paint viscosity and surface tension are not related to gun selection. These factors depend on the formulation of the paint. The increase in air pressure, which is a variable depending on the gun, is a factor that increases the atomization quality. The effect of nozzle diameter is less than pressure and can be used by changing it depending on the viscosity. The pressure of the paint is either constant in the fed vessels or depends on the pressure of the air in the others (gravity or suction feed). Therefore, in order to achieve good atomization at low pressure, it is necessary to increase the air volume. In this way, the high volume of air entering between the paint droplets expands with its exit to the outside environment and breaks the paint droplets into smaller droplets. The working principle of high volume low pressure (HVLP) air spray guns is based on these elements.
With the development of HVLP spray guns it has been possible to double the transfer efficiency. Paint is sprayed at high flow rate but at low pressure (with 0.2-0.7 atm air in HVLP systems instead of 2.5-5.5 atm in conventional systems). The fact that low air pressure greatly reduces the rebound effect leads to an increase in transfer efficiency. The prevalence of HVLP systems is increasing, especially in the field of refinish paints. In HVLP applications, an atomization and spreading quality close to that obtained in conventional applications can be achieved.
Figure 10. Comparison of HVLP and conventional guns in terms of rebound effect
The paint particles atomized with the HVLP gun are slightly bigger than the atomization in the conventional gun. Therefore, the surface quality obtained is expected to be worse than conventional applications. However, with recent studies, levels close to the surface quality applied with air systems can be achieved. However, the low air pressure prevents the paint particles from hitting the surface rapidly and rebounding (Figure-10). This, in turn, increases the application efficiency by 70-75%, resulting in savings in paint consumption.
Compared to conventional spray applications, a decrease is observed in the applications made with HVLP guns, in the amount of overspray dust going around and the amount of waste that causes environmental pollution. However, various restrictions have been introduced in some countries in order to prevent overspray dusts that may cause environmental pollution during the use of HVLP guns. For example; In some states of the USA and some European countries, the use of guns is prohibited, except for the HVLP, which has an air cap pressure of 0.7 bar. In addition, in the US state of Southern California, it is desirable to use HVLP guns that provide 65% or higher transfer efficiency at 0.7 bar and below pressure values.
In HVLP and conventional guns, the inlet pressure of the air to the gun and the outlet pressure in the air cap are given in Table-1. When the values in the table are examined, the transfer efficiency of HVLP guns is higher than conventional guns.
In mist-coat applications, it is not recommended to lower the pressure as in a conventional gun because the atomization quality will be adversely affected if the pressure drops too much. Instead, it would be more appropriate to increase the application distance.
Power of compressors used in HVLP guns; although it varies according to the manufacturer, gun quality, nozzle diameter and air cap, it is approximately between the values indicated in Table-3. This much power requires higher electricity consumption than conventional applications, but with HVLP application, savings in paint consumption are also provided.
Since the air volume used in HVLP guns is high, the inner diameter of the hose to be used should also be 0.9 cm (a conventional gun is around 0.8 cm). Therefore, when changing from a conventional gun to an HVLP gun, a user needs to change the gun, compressor and hose.
In HVLP applications, as in conventional applications, suction feed and gravity feed types of guns can be used. Very good transfer efficiencies cannot be obtained in suction feed HVLP spray guns. The reason for this is that the pressure of the air carrying the paint to the surface is low, sufficient vacuum cannot occur and as a result, the paint cannot be sprayed efficiently. In gravity feed type of guns, the transfer efficiency is better since gravity is used. The best efficiency is provided in pressure fed guns.